fix more casting away of qualifiers

The attached patches fix more cases where qualifiers (const, volatile)
are cast away either accidentally, or unnecessarily, or where it can be
worked around easily.

I split these into tiny bits to simplify review and to show that they
are all independent. But they could perhaps be committed all together.
(See also similar commit 3f988629805.)

I have a local WIP branch that fixes all remaining -Wcast-qual warnings.
The attached patches are the "easy" half of that. I plan to propose
addressing the other half separately later.